Conventionally, engine control devices are known, which include a plurality of cylinders, a valve stopping mechanism which, upon satisfaction of an execution condition of a reduced-cylinder operation in which operation of one or some of the plurality of cylinders are suspended, sets suspended cylinders from the plurality of cylinders and closes intake and exhaust valves of the suspended cylinders (e.g., among first to fourth cylinders, the first and fourth cylinders), and a controller which controls the valve stopping mechanism.
During the reduced-cylinder operation, by increasing an intake amount of the operating cylinders, an opening of a throttle valve is increased and a pumping loss is reduced. Therefore, a fuel consumption reducing effect can be expected.
Incidentally, when a misfire occurs in one of the operating cylinders during the reduced-cylinder operation, the number of operating cylinders which contribute to an engine output becomes less, and thus an engine speed significantly drops, which may cause an engine stop. Therefore, the reduced-cylinder operation is prohibited and an all-cylinder operation is resumed.
JP2006-132385A discloses an engine control device which includes a cylinder operation controlling module that switches an engine operation from an all-cylinder operation to a reduced-cylinder operation in which operation of one or some of the cylinders are suspended upon satisfaction of an execution condition of the reduced-cylinder operation, and a misfire detecting module which detects a misfire of the engine. When an engine speed drops more than a set value during the reduced-cylinder operation, the operation of the suspended cylinder(s) which reaches its ignition timing immediately after this engine speed drop is started to reliably prevent an engine stop due to a misfire.
Further, in a fluid transmission mechanism, such as an automatic transmission mechanism or a continuously variable transmission mechanism mounted on a vehicle, in order to reduce a torsional vibration of a drive system caused by a torque fluctuation of an engine, a centrifugal damper mechanism is used as a dynamic vibration absorber for attenuating the vibration.
As illustrated in FIG. 8A, the centrifugal damper mechanism which elastically deforms in its rotational direction to reduce vibration caused by a drive source when engaging a lockup clutch, is comprised of a plurality of damper springs 51 arranged on an outer circumferential side of the lockup clutch at an even interval in a circumferential direction of the centrifugal damper mechanism.
One circumferential end part of each damper spring 51 is directly supported by a receiving part 52a of a spring receiving member 52 extending radially outward from a clutch drum, and the other circumferential end part of the damper spring 51 is connected to a turbine hub and directly supported by a receiving part 53a of a spring retaining plate 53 for retaining an outer circumferential part of the damper spring.
Therefore, when torque is transmitted from the engine, a centrifugal force elastically deforms the damper spring 51 and moves the spring receiving member 52 in the rotational direction (see FIG. 8B).
However, with the misfire detecting device for the engine of JP2006-132385A, the misfire of the engine may erroneously be detected.
Normally, a misfire of an engine is detected based on an angular speed variation of the engine (crankshaft) according to a combustion stroke of each cylinder. Specifically, a number of times that the angular speed variation of the engine exceeds a given set value is counted and the misfire of the engine is detected when the count value exceeds a given threshold.
As a result of study, the present inventors found that, within a low engine load range in which the reduced-cylinder operation is performed, the output torque transmitted from the engine is low. Therefore, a so-called periodic movement phenomenon in which an elastically deforming speed of the damper spring 51 retained by the spring retaining plate 53 exceeds a rotational speed of the clutch drum and the circumferential end part of the damper spring 51 separates from the receiving part 52a of the spring receiving member 52 once and then collides therewith (see FIG. 8C) occurs, and this periodic movement phenomenon causes resonance of component(s) around the damper spring 51.
This resonance causes half-order vibration by the crankshaft similarly to the vibration caused by the angular speed variation of the engine when an actual misfire occurs, which results in detecting an angular speed variation larger than the set value similarly to when the engine misfire occurs. Thus, the engine misfire is erroneously detected even though it has not actually occurred.